The present invention relates generally to electrical machines and more specifically to wind turbine generators and ship propulsion motors.
Wind is usually considered to be a form of solar energy. Wind is caused by the uneven heating of the atmosphere by the sun, the irregularities of the earth's surface, and rotation of the earth, and wind flow patterns are modified by the earth's terrain, bodies of water, and vegetation. The terms wind energy or wind power describe the process by which the wind is used to generate mechanical power or electricity.
Typically, wind turbines are used to convert the kinetic energy in the wind into mechanical power. This mechanical power may be used for specific tasks (such as grinding grain or pumping water) or a generator may convert this mechanical power into electricity. A wind turbine usually includes an aerodynamic mechanism for converting the movement of air into a mechanical motion which is then converted with a generator into electrical power. Power output from the generator is approximately proportional to the cube of the wind speed. As wind speed doubles, the capacity of wind generators increases almost eightfold.
The majority of commercially available wind turbines utilize geared drive trains to connect the turbine blades to the wind generators. The wind turns the turbine blades, which spin a shaft, which feeds into a gear-box and then connects to a wind generator and makes electricity. The geared drive aims to increase the velocity of the mechanical motion. The drawback of a geared drive is that it reduces the reliability of the wind turbine and increases the noise and cost of the wind turbine.
A few wind turbines utilizing direct-drive generators are also commercially available. The large diameters of direct drive generators present formidable transportation and assembly challenges, both at the factories and at the wind turbine installation site. As the wind turbine industry matures and technology improves, larger power ratings will be required to continue the downward push in the cost of energy. Standard power ratings for land based turbines are expected to push to more than 3 MW in the next few years, and the offshore turbines are expected to be 5 MW or greater.
For the wind turbines and ship propulsion motors to evolve to higher power ratings, conventional approaches increase the direct-drive generator diameter or axial (stack) length. Increasing the diameter is preferred from a purely generator electromagnetic perspective, but is not attractive from the transportation, frame, and assembly perspective, especially for land-based turbines. Increasing the axial length of the generators, while maintaining the diameter to be less than approximately 4.1-4.3 meters, alleviates the land-based transportation issue, but results in complex and costly frame structures with long axial extents which presents a problem in that there is a limited amount of space available on board a ship. Also this also presents a problem of increased weight that reduces the efficiency of the motor in propelling a ship.
It is therefore desirable to provide cost-effective electrical machines of increased power ratings and reduced diameters.